1. Technical Field
The present disclosure relates to photodetectors, and in particular to single-photon avalanche photodiodes (SPAD).
2. Description of the Related Art
Photodetectors capable of detecting a single photon are used in many applications such as detecting an object and measuring distances, analyzing DNA or proteins, time-resolved spectroscopy such as fluorescence correlation spectroscopy and fluorescence life-time imaging, as well as inspecting VLSI high-density integrated circuits. Distance measurement can be carried out based on a propagation time of a beam of photons emitted in the form of pulses and reflected on the object.
One well-known method involves using photodiodes as detection and distance measurement elements, by using an avalanche phenomenon which may occur in the pn junction of photodiodes. An avalanche phenomenon can be triggered in a diode pn junction when the diode is reverse-biased in the vicinity of the breakdown voltage of the junction. This phenomenon can be used in two ways in an avalanche photodiode. If the avalanche photodiode is reverse-biased just below the breakdown voltage, the photodiode then generates an electric current proportional to the intensity of the flow of photons received by the photodiode, with a gain of a few hundred with a semiconductor such as silicon.
To detect low-intensity flows of photons, one well-known method involves using photodiodes which can be reverse-biased above the breakdown voltage. Such photodiodes are referred to as SPAD photodiodes or Single-Photon Avalanche Diodes or diodes operating in “Geiger” mode. Every time such a photodiode receives a photon, an avalanche phenomenon occurs in the pn junction of the photodiode, thus generating a relatively intense current. To prevent the photodiode from being destroyed by this intense current, the photodiode is connected to a quenching circuit enabling the avalanche process to be stopped a few nanoseconds after it occurred.
To perform a distance measurement, one well-known method involves lighting a detection zone with a pulsed light source such as a pulsed laser source, and detecting photons reflected by an object present in the detection zone using a detector comprising several SPAD photodiodes, for example disposed according to a matrix configuration. The distance of the object present in the detection zone is assessed on the basis of the propagation time or time of flight (TOF) between the instant a light pulse is emitted and the instant a pulse appears at the terminals of a photodiode, resulting from the avalanche triggering of the photodiode. The measurement accuracy depends particularly on the duration of the light pulses emitted by the source, and the shorter these pulses are, the more accurate the measurement can be.
In a CMOS-type integrated circuit, powered by a voltage in the order of 3 to 5V, the reverse bias of the SPAD photodiodes at a voltage higher than the breakdown voltage, requires a bias voltage of about 14V. Such a voltage is produced by a high voltage generating circuit, for example using a charge pump enabling the supply voltage to be increased.
It transpires that the breakdown voltage of a SPAD photodiode can vary greatly from one photodiode to another depending on the manufacturing conditions of the photodiodes. The breakdown voltage may also vary greatly over time particularly depending on the ambient temperature. Now, knowledge of this breakdown voltage is important to determine a minimum bias voltage enabling a SPAD photodiode to be put in condition for detecting a photon. Furthermore, the bias voltage of SPAD photodiodes must not be too high to avoid generating an excessively high so-called “dark current”. In addition, the higher this bias voltage is, the more leakage currents there will be in the circuits, and the more difficult the design of these circuits is.
In certain applications, it can also be desirable to place one or more SPAD photodiodes of a detector in a state in which they will not avalanche trigger under the effect of a photon. Now, cutting off a high voltage such as the bias voltage of SPAD photodiodes requires relatively large transistors that, on the other hand, have a relatively slow switch speed. If the detector comprises a large number of SPAD photodiodes that must be selectively biased, it is hardly conceivable to associate such a transistor with each SPAD photodiode. However, the bias voltage applied to each SPAD photodiode can be lowered below the breakdown voltage using small and fast transistors. However, this solution requires the breakdown voltage of each SPAD photodiode of the detector to be accurately known.
FIG. 1 represents a characteristic curve of current according to the bias voltage of a SPAD photodiode. The part of this characteristic curve corresponding to a negative bias voltage (reverse bias), comprises two parts C1, C2, respectively before and after the breakdown voltage Vbd of the SPAD photodiode. In the part C1 between 0V and the voltage Vbd, a reverse current substantially constant at a low value passes through the photodiode. In the part C2, beyond the voltage Vbd, the reverse current increases rapidly. FIG. 1 also represents a portion of curve C3 extending from the value of the current at the voltage Vbd to the negative currents and corresponding to the leakage currents in the SPAD photodiode. FIG. 1 also represents the bias voltage Vhv of the SPAD photodiode, the difference between the voltage Vhv and the breakdown voltage Vbd is noted Veb. The difference between the voltage Vhv and a voltage lower than the voltage Vbd, to ensure that the SPAD photodiode cannot avalanche trigger, is noted Vsd. The application of the voltage difference Vsd to the bias voltage Vhv prevents the SPAD photodiode from avalanche triggering.